Dry-developing photosensitive dry film resist

ABSTRACT

A dry-developing dry film resist is provided comprising a photopolymerizable layer preferably sandwiched between a support sheet and a cover sheet, the layer comprising polymerizable monomer in excess of the absorptive capacity of the layer, photopolymerization initiator, and binder component of a plurality of polymers, at least one of these polymers being incompatible so as to be present as a dispersion in the layer to reduce the cohesive strength of the layer. The layer is developed by peeling away the support sheet, the unexposed area of the layer adhering to the support sheet and the exposed area adhering to the substrate to which it was laminated to form a resist image against such treatments as etching, plating and soldering, especially to make a printed circuit.

This is a continuation of application Ser. No . 015,245 filed Feb. 26,1978, now abandoned.

TECHNICAL FIELD

This invention relates to a photosensitive resist material which iscapable of dry development to form a resist image on a surface.

BACKGROUND ART

The use of a photopolymerizable dry film as a resist for etching andplating a substrate to form a printed circuit is disclosed in U.S. Pat.No. 3,469,982. Although this use has gained widespread acceptance, ithas the disadvantage of requiring "wet" development of the resist image,i.e., the area of photopolymerizable layer that is unexposed in theimagewise photoexposure step, is removed by washing away with solventwhich does not dissolve the exposed area of the layer.

Numerous attempts have been made to make a photopolymerizable layer thatis developable dry, i.e., the unexposed area of the layer is removedfrom the exposed area of the layer merely by peeling away of the supportsheet, wherein the exposed area of the layer remains on the substrate towhich it was laminated and the unexposed area of the layer remainsadhered to the support sheet. None of these attempts have beencommercially successful. No dry-developing photopolymerizable dry filmresist is commercially available.

To detail some of these attempts, U.S. Pat. No. 3,770,438 discloses adry-developing photopolymerizable layer on a polymer film supportoperating according to the principle of the polymerizable monomer beingpresent in the layer in a quantity in excess of the absorptive capacityof the binder so that a thin layer of substantially free monomer ispresent on the surface of the photopolymerizable layer. This freemonomer is detectible on the surface of a substrate such as copper as anoily residue when the layer is peeled off of the copper prior to anyphotoexposure of the layer. The free monomer polymerizes whenphotoexposed to actinic radiation, to adhere the exposed area of thelayer to the copper more strongly than to the film support. The greateraffinity of the monomer for the film support than the copper in theunexposed area of the layer enables the unexposed area of the layer toremain adhered to the film support. Different ways for achieving anexcess of monomer are used in the patent, e.g., using a hydrocarbonpolymer binder and a very large proportion of monomer (77.5% in Ex. 3),using a chlorocarbon polymer binder having a lesser absorptive capacityfor the monomer, thereby reducing the monomer content to 40-50% (Ex. 9,13, 15 & 16), or using a combination of a chlorocarbon polymer (M. wt.about 20,000) and higher molecular weight (about 60,000)poly(methylmethacrylate) to decrease the flow of the layer. The amountof chlorocarbon polymer used ranged from 24.6 to 41.4 by wt., the amountof hydrocarbon polymer ranged from 3 to 29.4% by wt. and the amount ofmonomer ranged from 24.2 to 65.6% by wt. (Ex. 1, 2, 5-8, 12, 14, and16). The disadvantage of the dry-developing photopolymerizable layer ofthis patent was that either the flow of the layer at ordinary roomtemperature was too great or the fidelity of the image reproduced by thelayer was less than desired for the manufacture of printed circuits. Asthe flow of the photopolymerizable layer was reduced in an attempt toprovide a layer that could be supplied in the form of a roll, asrequired for commercial acceptance, the layer would either not failcohesively at the boundary between the unexposed area and exposed areaof the layer with a smooth, sharp break to give the image fidelitydesired or would not fail cohesively at all, i.e., even the unexposedarea of the layer would adhere to the substrate when the film supportwas peeled away. Layers that gave good image fidelity upon drydevelopment were so tacky that it was difficult to remove a coversheetdesired to be laminated to a surface of the layer. None of the Examplesuse a coversheet.

Japanese Patent Publication No. 35,722 published Sept. 28, 1978discloses earlier Japanese patent publications Nos. 43-22901 and47-7728; these correspond approximately to U.S. Pat. Nos. 3,353,955 and3,770,438 (mentioned above), respectively. According to Publication No.35,722, these earlier publications disclose a dry-developingphotopolymerizable layer composed of polymer binder, unsaturatedmonomer, and photopolymerization initiator, with the properties of thepolymer binder, such as molecular weight, softening point,crystallinity, miscibility with other materials, and adhesion towardsthe substrate as being important properties. Pub. No. 35,722 alsodiscloses that Japanese Patent Publication No. 38-9663 disclosespolyvinyl butyral, polyvinylacetate, polyvinylpyrrolidone, gelatin,coumarone indene resin, silicone resin, rubber, etc. and that Pub. No.47-7728 discloses binder of vinylidene chloride, cellulose ether,synthetic rubber, polyvinyl acetate copolymer, polyacrylate,polyvinylchloride, etc. Pub. No. 35,722 discloses that tests made ofphotopolymerizable compositions with these binders did not always obtainsatisfactory results. Pub. No. 35,722 purports to solve the problem byusing a chlorinated polyolefin, disclosing chlorinated polyethylene andchlorinated polypropylene as being preferred, as the polymer binder, inthe dry-developing photopolymerizable layer as a plating resist in whichthe resist is removed after plating and the substrate under the resistis etched.

U.S. Pat. No. 4,058,443 discloses the same background art as referred toin Pub. No. 35,722. U.S. Pat. No. 4,058,443 discloses further that themechanical strength of photopolymerizable layers used for drydevelopment are apt to have insufficient mechanical strength which isimportant when the layer is used as a tenting resist. U.S. Pat. No.4,058,443 purports to solve this problem by having the binder of thephotopolymerizable layer, in addition to chlorinated polyolefin, consistof 10 to 90% of a straight-chained polymeric material having sufficientcompatibility with the other components of the layer and having amolecular weight of at least 10,000, with a range up to 2,000,000 beingdisclosed. The chlorinated polyolefin is also claimed to have apolymerization degree of 600 to 20,000 which for chlorinatedpolypropylene (64% by wt. Cl) corresponds to a molecular weight of about34,000 to 2,200,000. The resist utility disclosed in this patent is asan etching resist. The layers of this patent are either too flowable orwhen not too flowable, they have deficient image fidelity upon drydevelopment. In addition, the conditions for peeling away the supportfilm to achieve dry development are too exacting for commercialpractice. As such these layers have nothing more than laboratoryapplication.

The need has also arisen for a particular type of dry film photoresist,namely one which is a mask for molten solder and which can remain as apermanent coating on the printed circuit. Such dry film solder maskwould offer greater image resolution and fidelity than silk-screenmethods now predominantly used. In addition to the usual requirements ofdry film photoresist used for plating or etching, the solder maskutility has the additional requirements that the dry film has toencapsulate the exposed surfaces of conductors, withstand the moltensolder without embrittlement or loss of adhesion to the printed circuitand without chemical degradation, and be flame retardent, moistureresistant, and a dielectric as well.

Belgian Pat. No. 860,181 granted Apr. 27, 1978 disclosesphotopolymerizable solder mask compositions provided as a layersandwiched between a film support and a coversheet. The layer islaminated to a printed circuit by application of heat and pressure andis imagewise exposed and then developed in the same manner as in U.S.Pat. No. 3,469,982, namely by "wet" development to obtain the soldermask (resist image) on the printed circuit. Layer thickness of from0.0008 cm (0.003 inch) to 0.025 cm (0.01 inch) is disclosed, but all theExamples use a layer thickness of 0.01 cm (0.004 inch) because that waswhat was required to encapsulate the conductors of the printedcircuitry. Conductors were often at least 0.005 cm in height requiringthicker solder mask layers in order to be able to encapsulate theconductors without excessive thinning out of the layer or puncture ofthe layer by the conductors.

U.S. Pat. No. 4,127,436 discloses a special vacuum lamination processfor applying the solder mask to the printed circuit, enabling thethickness of the photopolymerizable layer to be reduced. Example 1discloses layer thickness of 0.002 inch covering conductors of 0.002 to0.0044 inch in height. Encapsulation of the conductor by the solder maskis shown in FIG. 3. The solder mask used in this process were wetdeveloped but the concept of dry development is disclosed.

Photopolymerizable solder masks have been commercially available forseveral years first in the 0.01 cm thickness and eventually in the0.0075 cm thickness all for development by wet development.

A dry-developing dry film resist having commercial utility would bedesirable. To summarize the art discussed hereinbefore, thewet-developing dry film photoresist has achieved commercial utility andso has the wet-developing solder mask. Dry-developing dry filmphotoresists have been proposed and worked on but have not achievedcommercial utility. Dry-developing solder masks have been proposed butwithout any hint of how to achieve a commercially useful one. The needfor a commercially useful dry-developing resist film remainsunfulfilled.

DISCLOSURE OF INVENTION

The present invention satisfies the need for a dry-developing dry filmresist which has utility on a commercial basis as opposed to merelaboratory utility. The dry film resist can be described as follows:Dry-developing dry film resist for developing am image on a surface byremoval of unexposed areas from exposed area of said resist without theuse of solvent comprising a flexible support sheet and aphotopolymrizable layer having a greater adhesion to said support sheetthan to said surface, but upon exposure to actinic radiation, having agreater adhesion to said surface than to said support sheet, said layercomprising a plurality of polymeric binders, at least one of saidbinders being incompatible so as to be present as a dispersion in saidlayer to provide an appreciable reduction in the cohesive strength ofsaid layer as compared to when said incompatible binder is not presentin said layer, free-radical polymerizable monomer, andphotopolymerization initiator for initiating the polymerization of saidmonomer upon exposure of said layer to actinic radiation, said monomerbeing present in an amount in excess of the absorptive capacity of saidlayer, said layer being dry developed by lamination to said surface,imagewise exposure of said layer to actinic radiation to cause theexposed area of said layer to achieve said greater adhesion to saidsurface, and peeling away said support sheet, the reduced cohesivestrength of said layer enabling the unexposed area of said layer tobreak free from said exposed area and be removed therefrom by saidpeeling.

The dry film resist of the present invention is especially useful forthe demanding utility of a solder mask.

The present invention utilizes the principle of excess monomer of U.S.Pat. No. 3,770,438 but has found that image fidelity does not have to besacrificed as in this patent and U.S. Pat. No. 4,058,443 when layerflowability is reduced. The present invention provides a measure ofdecreased flowability, non-tackiness or low enough tack for easystripping of any coversheet, with high image fidelity by providing aheterogeneous photopolymerizable layer in which a disperse phase ofbinder polymer is present in the layer. This is in contrast to thecompatibility requirement disclosed in U.S. Pat. No. 4,058,443.

The layer is still flowable at elevated laminating temperatures so as tobe able to encapsulate the conductors of the printed circuit when usedas a solder mask, but the layer has sufficiently low flow at ordinaryroom temperature (20° C.) that the layer sandwiched between a filmsupport and cover sheet can be rolled up into a tight roll package of atleast 50 layers for handling and storage convenience that makes the dryfilm resist have commercial utility.

Unexpectedly, the dry film resist of the present invention while meetingall the requirements of a solder mask, e.g., resistance to molten solderand moisture resistance without embrittlemenet and being a dielectric,the photopolymerizable layer need be no more than 0.005 cm thick, tocover even higher conductors of a printed circuit. Preferably the layerthickness is no more than 0.0038 cm thick and more preferably to morethan 0.0025 cm thick.

Consequently, the solder mask of the present invention provides twoimportant advances over the dry film solder mask commercially availableheretofore, namely the elimination of wet development, and the decreaseof the layer thickness from 0.0075 cm to no more than 0.005 cm. For thedry film resist utility, in general, the resist of the present inventionprovides good image fidelity to the original image and sufficient roomtemperature viscosity (low flow) that the layer can be supplied inroller-up form.

The process of the present invention can be described as follows: Aprocess for selectively modifying a surface, comprising laminating thedry film resist of claim 1 to said surface, imagewise exposing saidlayer to actinic radiation, removing said support sheet from saidsurface, whereby the exposed area of said layer remains on said surfaceand the unexposed area of said layer remains on said support sheet to beremoved therewith, treating the resultant exposed area of said surfaceto modify it, the exposed area of said layer protecting the remainder ofsaid surface from said treating.

FURTHER DESCRIPTION OF INVENTION AND BEST MODE

The present invention will be described in detail with reference to thesolder mask utility. It should be recognized, however, that resist whichwithstands molten solder will also serve as a resist to other depositionof metal such as plating or to etching.

The photopolymerizable layer used in the dry film resist of the presentinvention is made of binder, monomer, and photoinitiators which bear acertain relationship to each other and to the layer in toto, and thesecomponents are combined into the layer in a special way to bring aboutthe dispersed phase of incompatible binder relationship.

To first discuss the layer formulation process, the components are addedto solvent being stirred at room temperature. Preferably sufficientsolvent is present, i.e., solids concentration is low enough, that thestirring produces a solution of the components, thereby insuringcomplete mixing thereof. When stirring is stopped, the liquid medium ishazy in appearance. This is best visible to the naked eye when nocolorant is in the medium. If excessive solvent is present, some of itwill require evaporation before the haze develops.

If the medium is left standing for a still longer time, the hazecoalesces to form a stratified separate phase in the medium. It has beenfound that the amount of incompatible binder present in the compositionin the medium, increases (the rate of separation being increased withthe help of centrifuging) the amount of stratified separate phase,thereby indicating that the incompatible phase is of incompatiblepolymer (including a phase at least rich in such polymer).

The solvent for the medium is selected so that the phase separation isnot so rapid that stratification occurs before or when the medium iscoated on a support sheet and dried to form the photopolymerizablelayer. For the composition of Example 1 for example, methylene chloridehas been found to be a satisfactory solvent while methyl chloroform andmixtures of cellosolve and methylene chloride are unsatisfactory.

It is not required that the hazy appearance in the liquid medium beseen. It is only required that the dispersed incompatible binder phaseforms in the layer during drying.

The dispersed phase is visible in the photopolymerizable layer thrugh aphase contrast microscope for example at about 100X magnification.Several different forms of dispersion have been observed, namely, adispersion of minute, discrete, spherical-like domains in a continuousphase. The preferred dispersion has the appearance of a mixture ofelongated (ribbon-like) domains of different phase with no clearcontinuous phase. The size of the domains are such that the layer has ahazy appearance which may be visible with the naked eye if the layer isnot too deeply colored. The clear visibility of the domains in themicroscope indicates the size of the domains are greater than colloidalin size.

The dispersed phase functions in the layer by reducing the cohesivestrength of the layer. Cohesive strength is tested by the amount offorce needed to peel away the support sheet from the photo-exposed areaof the layer adhered to a surface. The force is measured using a tensilestrength tester (Instron), and the cohesive strength measured is theforce needed to break the thickness of the layer at the boundary betweenthe photo-exposed area of the layer and the unexposed area thereof.

This reduction in cohesive strength of the layer is believed to becaused by the cohesive break occurring along interfaces between theincompatible phases or by fracture of the incompatible phase at theboundary between exposed and unexposed areas. Where the incompatiblephase is present as discrete domains in the layer, the presence of thesedomains shortens the path length of the cohesive failure within thecontinuous phase of relatively high cohesive strength, from discretedomain to discrete domain to appreciably reduce the cohesive strength ofthe layer. This requires that the adhesion between the phases is lowrelative to the cohesive strength of the layer without the incompatiblepolymeric binder. Preferably, a reduction in cohesive strength of atleast 25%, and more preferably at least 50%, and even more preferably atleast 75% is obtained, as compared to when no incompatible polymerbinder is present, provided that the layer (unexposed area) still hassufficient cohesive strength to be removed from the surface to which itis laminated.

With respect to the composition of the photopolymerizable layer, theplurality of binders are selected to produce the incompatibilityhereinbefore described and to impart a relatively no flow condition tothe layer at ordinary room temperature (20° C). This incompabilityresults from chemical dissimilarity of the binder polymers and/ordifferences in molecular weight to produce a lack of affinity of atleast one polymeric binder to another polymeric binder of the bindercomponent.

With these criteria in mind, it is apparent that numerous combinationsof polymers as the binder component are possible. Preferably, the majorproportion, i.e., greater than 50% by wt. of the binder component ischlorine-containing polymer because such polymer also has a limitedabsorptive capacity for the preferred acrylate polymerizable monomer.Preferably, the weight average molecular weight of all polymeric bindersis at least 10,000. Examples of chlorine-containing polymers arepolyvinylchloride, polychloroprene, polychlorobutadiene and chlorinatedpolyolefins such as chlorinated polyethylene, chlorinated polypropylene,and chlorinated polyisoprene. Preferably, the chlorine content of thepolymer is at least 40% by weight.

Preferably, the chlorine-containing polymer binder consists of aplurality of such polymers of different molecular weights, selected toinclude a component of relatively low molecular weight so as to providegood image fidelity and a higher molecular weight component so as toreduce the flow of the layer at ordinary room temperature, whichunfortunately also increases the cohesive strength of the layer. By wayof example, the low molecular weight range can be from 10,000 to 30,000weight average molecular weight and the higher molecular weight rangecan be from 40,000 to 75,000 weight average molecular weight. Apreferred combination is chlorinated polyisoprene in the low molecularweight range and chlorinated polypropylene in the higher molecularweight range. The higher molecular weight polymer preferably constitutesfrom 65 to 90% of the weight of the total chlorine-containing polymerand the low molecular weight chlorine-containing polymer from 10 to 35%of the weight of the total chlorine-containing polymer.

The incompatible polymer can be one or more such polymers. Preferably,the polymer is an organic polymer, which is substantially free ofchlorine, more preferably an alkyl acrylate polymer in which the alkylgroup contains from 2 to 8 carbon atoms. Polymethylmethacrylate has beenfound not to produce a dispersed phase in a layer containing thechlorine-containing polymer and not to give the desired image fidelityat a room temperature flow condition which is useful commercially.Examples of poly(alkyl acrylate) polymers are poly(n-butyl methacrylate)preferably having a weight average molecular weight greater than100,000, a copolymer of 67% by wt. methyl methacrylate/30% by wt.2-ethyl hexylacrylate/2% methacrylic acid (the alkyl groups average outto an alkyl equivalent of at least 2 carbon atoms), preferably having aweight average molecular weight of 25,000 to 50,000, and poly (ethylmethacrylate), preferably having a weight average molecular weight of atleast 100,000. Preferably, the incompatible polymeric binder has a glasstransition temperature (Tg), as measured by differential scanningcalorimetry, of less than 70° C., which is characteristic of theabove-mentioned specific polymers. It is desirable that the incompatiblepolymer be soluble in solvent which dissolves the other ingredients ofthe composition used to make the layer.

The proportion of incompatible polymer to total polymeric binder willgenerally be within the range of 2 to 45% by wt., more often 15 to 35%by wt., depending on the particular polymers present in the polymericbinder in order to get the dispersed phase of incompatible polymer inthe layer.

The essential remaining components of the layer are the monomer andphotoinitiator. The monomer preferably has at least one terminalethylenically unsaturated group and is liquid at ordinary roomtemperature and has a boiling point in excess of 100° C. Examples ofmonomers include one or more of the following monomers: pentaerythritoltriacrylate, trimethylolpropane triacrylate, ethylene glycol diacrylateand dimethacrylate, diethylene glycol diacrylate and other monomersdisclosed in U.S. Pat. No. 3,770,438. The amount of monomer present inthe layer is an amount in excess of the absorptive capacity of thelayer, as indicated by a monomer-rich phase being present on a coppersurface to which the layer is laminated and then delaminated (withoutphotoexposure) after letting stand for a reasonable period of time, e.g.two hours. Since the polymer binder component of the layer is thecomponent possessing absorptive capacity for the monomer, then themonomer is present in an amount in excess of the absorptive capacity ofthe polymeric binder component. Each polymeric binder has its ownabsorptive capacity for the monomer, so the amount of monomer requiredwill depend on the polymer present as the binder component as well as onthe type of monomer. The proportion of incompatible polymer, e.g. shouldnot be so high that while it reduces cohesive strength, it soaks up theexcess monomer to cause the exposed image to lose adhesion to thesurface to which it is laminated.

Example of photoinitiators include Michler's ketone, benzophenone,9,10-phenanthrene-quinone, 2-t-butylanthraquinone, and combinations ofhexaarylbisimidazoles and leuco dyes. The photoinitiator can consist ofone or more components which act separately or in combination toinitiate polymerization of the monomer.

The layer preferably contains a plasticizer for one or more of thepolymeric binders. Examples of plasticizers include mixed ester oftriethylene glycol dicaprate and dicaprylate, chlorinated paraffin andcetyl ether of polyoxyethylenealcohol. The plasticizer increases theelevated temperature flowability of the layer without making the flow atordinary room temperature too high.

Typically, the photopolymerizable layer will contain from 40 to 65% bywt. polymeric binder, and preferably 45 to 60% by wt.; 35 to 60% by wt.monomer, and preferably 40 to 50% by wt.; 1 to 10% by wt. plasticizer,and preferably 2 to 8% by wt.; and 1 to 10% by wt. of photoinitiator,and preferably 2 to 8% by wt.

Other ingredients can be present in the layer such as an inhibitor ofthermally-induced polymerization and a colorant.

The photopolymerizable layer is made into dry film resist by solutionblending of the hereinbefore described components, coating the solutiononto a support sheet, preferably polyethylene terephthalate whichprovides dimensional stability to the layer after drying, drying thesolution to form a layer on the support sheet, preferably followed bylaminating a coversheet, preferably of polyethylene, to the exposedsurface of the layer.

The dry layer is of uniform thickness and has sufficient viscosity (noflow) that the layer can be wound up under sufficient tension that theresultant roll does not telescope when held vertically with the core forthe roll unsupported. Typically, such roll will have at least 100 layersof photopolymerizable layer and more often at least 300 layers thereof.

In use, the cover sheet is removed and the layer is laminated to thesubstrate surface desired, usually a printed circuit consisting of apattern of conductors on a dielectric surface usually of glass-filledthermosetting resin. The lamination is conducted so that the layer flowsto encapsulate the conductors without excessive thinning of the layer orpuncture of it by the conductors. Next, the layer is imagewise exposedto actinic radiation, with the unexposed area of the layer correspondingto area of the printed circuit where soldering is desired. The next stepis to peel away the sheet support, preferably at room temperature,whereby the unexposed area of the layer is removed with it, leaving theexposed area of the layer as a dielectric permanent coating adheringfirmly to both the metallic and dielectric areas of the printed circuit.The resultant exposed area of the printed circuit is then contacted withthe molten solder (after removal of the support sheet) to coat theexposed area of the printed circuit with solder. This step can be used,e.g., to solder electrical components to the circuit board.

Instead of using the resist as a solder resist, the dry film resist ofthe present invention can be used as a resist in conventional etchingand/or plating operations.

The dry film resists of the present invention provide high circuit linequality (minimal raggedness of line edges), high resolution (ability todevelop small solder pad sizes and spaces between resist where solder isto be applied), low peel force, and a wide latitude in peel rate, e.g.,5 to 50 inches/min. at room temperature.

Examples of the present invention are disclosed hereinafter. In theseExamples, parts and percents are by weight unless otherwise indicated.Also the acrylate polymer is present in the layer as the dispersedphase.

EXAMPLE I p A photopolymerizable composition was prepared with thefollowing ingredients:

    ______________________________________                                        Poly(N-Butyl Methacrylate)                                                                             4g.                                                  wt. av. mol. wt. 200,000                                                      Chlorinated Polyisoprene,                                                                              12g.                                                 wt. av. mol. wt. 19,000 about                                                 65% by wt. Cl                                                                 Chlorinated Polypropylene,                                                                             6g.                                                  mol. wt. 62,000, about                                                        65% by wt. Cl.                                                                Mixed ester of triethylene                                                                             3g.                                                  glycol dicaprate and                                                          triethylene glycol                                                            dicaprylate                                                                   Pentaerythritol triacrylate                                                                            20g.                                                 1,4,4-Trimethyl-2,3-diazo-                                                                             0.025g.                                              bicyclo [3 . 2 . 2] non-2-                                                    ene n,n.sup.1 -dioxide                                                        Benzophenone             0.6g.                                                Michler's Ketone         0.06g.                                               Victoria Green dye       0.06g.                                               Tris-(4-diethylamino-O-tolyl)                                                                          0.5g.                                                methane mol.wt. 499 melt.                                                     point 168-170° (leuco dye)                                             Methylene Chloride       150g.                                                ______________________________________                                    

The ingredients were thoroughly mixed for four (4) hours and then coatedonto 0.00254 thick transparent polyethylene terephthalate film using a0.0127 cm coating doctor knife. During coating and drying to a drythickness of 0.00125 cm, the polyethylene terephthalate film wasfastened to a rigid support. When dry, the layer had no room temperatureflow such that a coversheet could be laminated to the layer and theresultant sandwich rolled up. This was not done, however, because thelayer was being used immediately. Thus, the layer was laminated to aprinted circuit board having an epoxy resin-based surface with 0.0050 to0.0075 cm circuit lines using a hot roll (100° C.) hard rubber rolllaminator. The resulting element was then exposed imagewise to a 2,000watt mercury vapor lamp UV source for 45 seconds through the transparentpolyethylene terephthalate support sheet. The photoprinted image wasthen dry developed by removing the polyethylene terephthalate supportsheet by hand peeling at a 180° angle at room temperature, whereby theunexposed areas of the image were removed from the layer by adhering tothe support sheet and the exposed areas of the layer were retained onthe board to form a solder mask. The printed circuit board with the maskis then cured thermally in an oven for one hour at 150° C., and then runthrough a wave-solder process at 244° C. using flux. The mask showedexcellent adhesion to the copper circuit lines and the epoxy resinsurface through the operation. The printed circuit board was thencleaned with a flux-removing solvent, methylchloroform, to leave thesolder mask as a permanent flame, chemical, and moisture resistantdielectric photopolymer coating on the board.

The process was repeated using circuit boards having both gold andsolder on the surface prior to applying the photopolymer layer.Satisfactory results were obtained in all cases.

This element tested for flammability using UL-94 test proceduresexhibited V-O rating under repeated testing.

EXAMPLE II

The same photopolymerizable composition was prepared as in Example I.This was then extrusion coated onto 0.00254 cm polyethyleneterephthalate and dried to give a 0.00165 cm photopolymer layer. Afterthis application, a polyethylene coversheet was laminated on thephotopolymer to give a sandwich structure. It could then be stored inroll form until use. To forestall long-term sideways flow of the layer,the layer can be edge-hardened by the process disclosed in U.S. Pat. No.3,867,153. After removal of the protective polyethylene cover sheet, theelement was then laminated to the printed circuit board as in Example Iand when so tested gave the same results as indicated in Example I.

EXAMPLE III

Results equivalent to Example I were obtained when the copolymer ofmethyl methacrylate/ethyl hexyl acrylate/methacrylate acid (weightaverage molecular weight of 30,000 to 40,000) hereinbefore described wassubstituted for poly(n-butyl methacrylate) on a weight for weight basis.

EXAMPLE IV

A photopolymerizable composition providing similar results was preparedas in Example I except that a high molecular weight soybean oilpolyester polymeric epoxide containing 65% oxygen by weight was used asa plasticizer instead of the mixed ester of triethylene glycol dicaprateand triethylene glycol diaprylate.

EXAMPLE V

A photopolymerizable composition was prepared from the followingingredients:

    ______________________________________                                        Poly(ethyl methacrylate) 3g.                                                  wt. av. mol. wt. 250,000                                                      Chlorinated polyisoprene 19g.                                                 wt. av. mol. wt. 19,000,                                                      about 65% by wt. Cl.                                                          Mixed ester of triethylene glycol                                                                      3g.                                                  dicaprate and dicaprylate                                                     Pentaerythritol triacrylate                                                                            20g.                                                 Benzophenone             1.6g.                                                Michler's Ketone         0.2g.                                                Rhodamine BI base dye    0.05g.                                               Methylene chlordide      150g.                                                ______________________________________                                    

This composition was coated and heat treated as in Example I to producea solder mask with good room temperature peel-apart properties.

EXAMPLE VI

A satisfactory solder mask formulation was prepared using 0.1 g ofbenzotriazole in addition to the ingredients in Example V.

EXAMPLE VII

A photopolymerizable composition was prepared with the followingingredients:

    ______________________________________                                        Poly(n-butyl methacrylate)                                                                             3.5g.                                                wt. av. mol. wt. 200,000                                                      Chlorinated polyisoprene 18.5g.                                               wt. av. mol. wt. 19,000,                                                      about 65% by wt. Cl.                                                          Pentaerythritol triacrylate                                                                            20g.                                                 Mixed ester of triethylene                                                                             3g.                                                  glycol dicaprate and                                                          triehtylene glycol dicaprylate                                                2,2'-methylene bis(4-ethyl-6-                                                                          0.5g.                                                tert-butyl phenol)                                                            Benzophenone             1.6g.                                                Michler's Ketone         0.2g.                                                Victoria Green Dye       0.05g.                                               Methylene Chloride       150g.                                                ______________________________________                                    

This composition was coated and heat treated as in Example I to producea solder mask with good room temperature peel-apart properties.

EXAMPLE VIII

Equivalent solder mask results were obtained when a copolymer ofethylacrylate and n-butyl methacrylate mol. wt. 30,000 to 40,000 wassubstituted for n-butyl methacrylate in the composition of Example VII.

EXAMPLE IX

Equivalent results for solder mask were obtained when a terpolymercomprising 67% methyl methacrylate/31% 2-ethylhexylacrylate/2%methacrylic acid of mol. wt. 30,000 to 40,000 was substituted forn-butyl methacrylate in the composition for Example VII.

EXAMPLE X

A photopolymerizable composition was prepared with the followingingredients:

    ______________________________________                                        Terpolymer 65% methyl   4g.                                                   methacrylate/31% 2-ethyl                                                      hexyl acrylate/2% meth-                                                       acrylic acid, wt. av.                                                         mol. wt. 30,000 to 40,000                                                     Chlorinated polyisoprene                                                                              18g.                                                  wt. av.mol. wt. 20,000,                                                       about 70% by wt. Cl.                                                          Pentaerythritol triacrylate                                                   Mixed ester of triethylene                                                                            3g.                                                   glycol dicaprate and                                                          triethylene glycol                                                            dicaprylate                                                                   Benzophenone            1.6g.                                                 Michler's Ketone        0.2g.                                                 Victoria Green Dye      0.05g.                                                Methylene Chloride      150g.                                                 ______________________________________                                    

The composition was coated and heat treated as in Example I to produce asolder mask with good room temperature peel-apart properties.

EXAMPLE XI

Equivalent solder mask results were obtained when the formulation ofExample I was altered by substituting Victoria Pure Blue Dye forVictoria Green and also adding 5 g. of a chlorinated paraffin (60% bywt. Cl.).

EXAMPLE XII

Equivalent solder mask results were obtained when the formulation ofExample XI contained 5 g. of cetyl ether of polyoxyethylene alcoholinstead of the chlorinated paraffin.

EXAMPLE XIII

A photopolymerizable composition was prepared with the followingingredients:

    ______________________________________                                        Chlorinated paraffing    5g.                                                  (70% by wt. Cl.)                                                              Poly(n-butyl methacrylate)                                                                             2g.                                                  wt. av. mol. wt. 200,000                                                      Chlorinated polyisoprene 14g.                                                 wt. av. mol. wt. 19,000, 14g.                                                 about 65% by wt. Cl.                                                          Chlorinated polypropylene                                                                              6g.                                                  wt. av. mol. wt. 60,000,                                                      about 65% by wt. Cl.                                                          Pentaerythritol tricrylate                                                                             20g.                                                 Mixed ester of triethylene glycol                                                                      3g.                                                  dicaprate and triethylene                                                     glycol dicaprylate                                                            1,4,-Trimethyl-2,3-diazobicyclo                                                                        0.025g.                                              [3 · 2 · 2] Non-2-ene N,N'-dioxide                          Benzophenone             1.6g.                                                Michler's Ketone         0.2g.                                                Victoria Pure Blue Dye   0.03g.                                               Methylene Chloride       150g.                                                ______________________________________                                    

This composition was coated and heat treated as in Example I to producea solder mask with good room temperature peel-apart properties.

EXAMPLE XIV

This example shows the effect of varying the amount of incompatiblepolymeric binder in a photopolymerizable layer on the appearance of thedispersion, peel force, and resolution.

A series of solutions of the following photopolymerizable compositionswas prepared as in Example I.

    ______________________________________                                                  (wt. in g.)                                                                   A    B      C      D    E    F    G                                 ______________________________________                                        Ch.sub.2 Cl.sub.2                                                                         265    265    265  265  265  265  265                             Leuco Dye   1.5    1.5    1.5  1.5  1.5  1.5  1.5                             Poly        0      3      6    9    12   15   18                              (n-butyl                                                                      methacryl-                                                                    ate of                                                                        Example I                                                                     Plasticizer 9      9      9    9    9    9    9                               of                                                                            Example I                                                                     Thermal     0.075  0.075  0.075                                                                              0.075                                                                              0.075                                                                              0.075                                                                              0.075                           Inhibitor                                                                     of                                                                            Example I                                                                     Victoria    0.18   0.18   0.18 0.18 0.18 0.18 0.18                            Green                                                                         Benzophenone                                                                              4.8    4.8    4.8  4.8  4.8  4.8  4.8                             Michler's   0.6    0.6    0.6  0.6  0.6  0.6  0.6                             Ketone                                                                        Chlorinated 44     42     40   38   36   34   32                              Poly-                                                                         isoprene                                                                      of                                                                            Example I                                                                     Chlorinated 22     21     20   19   18   17   16                              Poly-                                                                         propylene                                                                     of                                                                            Example I                                                                     Triacrylate 60     60     60   60   60   60   60                              Monomer                                                                       of                                                                            Example I                                                                     ______________________________________                                    

The solutions were coated onto polyethylene terephthalate film and driedto form a dry layer thickness of 0.00165 cm as in Example I.

1. To observe the dispersed incompatible phase present in the layer,each sample prepared in the preceding paragraph was laminated to amicroscope slide and photoexposed as in Example I and then thepolyethylene terephthalate film was peeled away leaving the entirephoto-exposed layer on the microscope slide. The microscope slide wasobserved at 106X in a phase contrast microscope. The photo-exposed layerof sample A had the appearance of a single phase. The photo-exposedlayer of samples B, C, and D exhibited globules of a second phasedispersed in a continuous phase of increasing amounts (of domains) fromsample B to sample D, the size of the domains being similar in allsamples. The photo-exposed layer of sample E exhibited a differentappearance, namely instead of a dispersion of domains in a continuousphase, the dispersion had the appearance of a dispersion of domains in adispersion of domains of a different phase, each domain appearing to beribbon-like in appearance. The photo-exposed layer of samples F and Ghad the appearance of sample E plus the presence of spherical domainsdispersed therein.

2. To measure peel force, a sample of each phase was laminated to cleancopper-clad epoxy laminate using a vacuum laminator at a temperature of100° C. The laminate was sheared into 2.54 cm wide strips. A strip fromeach sample was photo-exposed as in Example I through a processtransparency containing a line target oriented so the lines ranwidthwise (perpendicular to the length) of the strip. The strip wasclamped in a tensile tester (Instron) and the polyethylene terephthalatesupport sheet was peeled off lengthwise of the strip, for about one halfthe length of the strip at a peeling rate of 12.7 cm/min. and theremaining length at a peeling rate of 50.8 cm/min., all at roomtemperature. The highest force required to peel off the support sheetand thereby develop the image (exposed) on the copper surface, is theforce required to achieve cohesive failure (break) at the trailing edgeof the exposed image (lines). The forces were as follows:

    ______________________________________                                                Sample                                                                Peel Rate                                                                             (peel force (g) along 2.54 cm wide of strip)                          (cm/min.)                                                                             A       B      C     D    E     F    G                                ______________________________________                                        12.7    120     100    110   40   20    46   50                               50.8     60      50     55   32   18    36   40                               ______________________________________                                    

These results show a minimum peel force for both peel rates for sampleE, providing a reduction in peel force of greater than 80% as comparedto sample A which does not contain any incompatible polymeric binder.These results also show a minimum in peel rate sensitivity for sample E,i.e., the least variation in peel force with changing peel rate. Thisgives processing latitude to the use of the layer.

3. To measure resolution, a strip of each sample laminated tocopper-clad epoxy laminate prepared in the preceding paragraph wasphoto-exposed as in Example I through a process transparency containinga pattern of opaque dots ranging in diameter from 0.005 to 0.038 cm. Thepolyethylene terephthalate support sheet was peeled away from the coppersurface at a constant peel rate of 50.8 cm/min. to develop the exposedimage on the copper surface in which dots of exposed copper surface werethen visible. The diameter of the smallest dots of unexposed layerremaining on the support sheet was measured. This experiment wasrepeated except that a converging line pattern in the processtransparency was used and the most closely spaced lines/spaces whichwere separated in the image resulting from peeling of the support sheetwere measured and recorded as follows:

    __________________________________________________________________________             Sample                                                                        A   B   C   D   E   F   G                                            __________________________________________________________________________    Dot      0.076                                                                             0.102                                                                             0.076                                                                              0.063/                                                                            0.038/                                                                            0.102/                                                                           0.063                                        Resolution           0.070                                                                             0.05                                                                              0.076                                            (cm)                                                                          Line      0.025/                                                                            0.038/                                                                           0.025                                                                              0.020/                                                                            0.015/                                                                            0.023/                                                                           --/                                          Resolution                                                                             0.051                                                                             0.051   0.025                                                                             0.018                                                                             0.030                                                                             0.030                                        (cm)                                                                          Line/Space                                                                    __________________________________________________________________________

Samples F and G suffered from some of the exposed image lines on thecopper being lifted off of the copper by the peeling away of the supportsheet; this lift-off was severe for sample G. This indicates monomer wasnot present in excess, having been absorbed by the additional acrylatepolymer, whereas adhesion of the exposed areas in this example and theother examples disclosed herein indicates the presence of excessmonomer.

4. The solutions prepared as set forth in the first paragraph of thisexample except for omission of the dyes were centrifuged and theproportion of separate stratified (upper) phase was estimated as followsas a % of the total liquid volume:

    ______________________________________                                                Sample                                                                        A     B     C       D   E     F   G                                   ______________________________________                                        Stratified                                                                              0       13    20    25  30    40  55                                Phase (%)                                                                     ______________________________________                                    

We claim:
 1. Dry-developing dry film resist for developing an image on asurface by removal of unexposed area from exposed area of said resistwithout the use of solvent comprising a flexible support sheet and aphotopolymerizable layer having a greater adhesion to said support thanto said surface, but upon exposure to actinic radiation, having agreater adhesion to said surface than to said support sheet, said layercomprising (1) a dispersion of at least one polymer having a Tg below70° C. in the form of spherical-like domains within a continuous phaseof (2) at least one further polymer, the adhesion between the domains ofthe dispersed polymer and the further polymer being low relative to thecohesion of the continuous phase of second polymer, to provide anappreciable reduction in the cohesive strength of said layer as comparedto when the dispersed polymer is not present in said layer, (3)free-radical polymerizable monomer, (4) and photopolymerizationinitiator for initiating the polymerization of said monomer uponexposure of said layer to actinic radiation, said monomer being presentin an amount in excess of the absorptive capacity of said layer, saidlayer being dry developed by lamination to said surface, imagewiseexposure of said layer to actinic radiation to cause the exposed area ofsaid layer to achieve said greater adhesion to said surface, and peelingaway said support sheet, the reduced cohesive strength of said layerenabling the unexposed area of said layer to break free from saidexposed area and be removed therefrom by said peeling.
 2. The dry filmresist of claim 1 having sufficient viscosity to be wound up into atight roll without loss of thickness of said layer.
 3. The dry filmresist of claims 1 or 2 and additionally a coversheet adhered to thesurface of said layer opposite the surface to which said support sheetis adhered, the adhesion of said coversheet to said layer being lessthan the adhesion of said support sheet to said layer.
 4. A tight rollof at least 50 layers of the dry film resist of claim
 3. 5. The dry filmresist of claim 1 having a thickness up to 0.005 cm.
 6. The dry filmresist of claim 1 wherein said layer is laminated to said surface whichincludes copper.
 7. A solder mask for dry development of a polymerizedimage on selected areas of a surface, especially a printed circuitboard, to protect said selected areas from molten solder and beingcapable of also acting as a permanent dielectric coating for saidselected areas, comprising a flexible support sheet and aphotopolymerizable layer having a greater adhesion to said support sheetthan to said surface, but upon exposure to actinic radiation, having agreater adhesion to said surface than to said support sheet, said layercomprising (1) a dispersion of at least one polymer having a Tg below70° C. in the form of spherical-like domains within a continuous phaseof (2) at least one further polymer, the adhesion between the domains ofthe dispersed polymer and the further polymer being low relative to thecohesion of the continuous phase of second polymer, to provide anappreciable reduction in the cohesive strength of said layer as comparedto when the dispersed polymer is not present in said layer, (3)free-radical polymerizable monomer, (4) and photopolymerizationinitiator for initiating the polymerization of said monomer, saidmonomer being present in an amount in excess of the absorptive capacityof said layer, the polymerized image on said surface formed bylaminating said layer to said surface, image-wise exposing said layer toactinic radiation and peeling away said support sheet to develop saidimage being resistant to molten solder and a dielectric.
 8. The soldermask of claim 7 wherein said incompatible binder is poly(alkyl acrylate)wherein the alkyl group contains from 2 to 8 carbon atoms.
 9. The soldermask of claim 7 wherein said monomer is an acrylate monomer.
 10. Thesolder mask of claims 7 and 8 wherein at least one of the other of saidpolymer binders is a chlorine-containing polymer.